In the past decade, apoE has emerged as one of the best validated risk factors for late-onset, sporadic Alzheimer's disease (AD). Despite a great deal of research that has significantly improved understanding of apoE and its receptors, the mechanism by which apoE genotype influences an individual's predisposition to AD remains unknown. We know that specific lipoprotein receptors are essential in maintaining normal synaptic plasticity and learning and memory processes in the adult mouse hippocampus. The ligand- receptor interaction between apoE and its receptors is well poised in the molecular framework of the synapse to have broad implications for both normal cognitive processes and the perturbations observed in early AD. The overall hypothesis of this proposal states that that apoE acts as an isoform-specific signaling ligand to modulate neuronal synaptic plasticity and hippocampal-dependent memory formation, and is susceptible to changes in fi amyloid accumulation. The different apoE isoforms, the ligand reelin and the four prominent apoE receptors that bind these ligands adds an exceedingly complicated level of complexity to this system. This proposal is designed to better understand four important aspects of apoE signaling and apoE receptor function: 1) The circumstances that influence apoE receptor processing. 2) The mechanisms that underlie apoE-dependent changes in synaptic function and memory formation. 3) The interactions between specific apoE receptors and apoE isoforms in receptor signaling and processing. 4) The role of apoE isoform signaling and apoE receptor processing in the pathological processes associated with Alzheimer's disease. These studies will be the first to identify interactions of apoE isoforms to specific receptors and how those interactions can affect CNS function. These insights will be valuable in assessing AD risk, formulating new treatment strategies for AD, identifying potential therapeutic drug targets for the management of AD and provide insight into other age-related disorders involving the lipoprotein receptor system.